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Free keywords:
General Relativity and Quantum Cosmology, gr-qc,High Energy Physics - Theory, hep-th
Abstract:
It was recently shown by Feldbrugge et al. that the no-boundary proposal,
defined via a Lorentzian path integral and in minisuperspace, leads to unstable
fluctuations, in disagreement with early universe observations. In these
calculations many off-shell geometries summed over in the path integral in fact
contain singularities, and the question arose whether the instability might
ultimately be caused by these off-shell singularities. We address this question
here by considering a sum over purely regular geometries, by extending a
calculation pioneered by Halliwell and Louko. We confirm that the fluctuations
are unstable, even in this restricted context which, arguably, is closer in
spirit to the original proposal of Hartle and Hawking. Elucidating the reasons
for the instability of the no-boundary proposal will hopefully show how to
overcome these difficulties, or pave the way to new theories of initial
conditions for the universe.
